Recently, several studies have been conducted to improve rate control (RC) optimization in High Efficiency Video Coding (HEVC). There are three categories of RC algorithms for HEVC: quadratic model [1], ρ-domain model [2] and R-λ, model. More specifically, Li et al. [3] first proposed the λ domain RC based on the relationship between coding bits and the Lagrangian multiplier. Due to the low complexity and high efficiency, the R-λ, model has been adopted in HEVC reference software as the default RC algorithm. Lee et al. investigated the Laplacian probability distribution function (PDF) in [4] to model the residue and proposed independent R-Q models to establish the relationship between the quantization parameters and coding bits, including texture and non-texture bits. Moreover, intra frame RC algorithms have also been studied. Li et al. [5] proposed an adaptive bit allocation algorithm to improve the R-λ, model RC algorithm on intra frame. In [6], sum of absolute transformed differences (SATD) was used to measure the complexity for intra-frame, which further improves the performance. Wang et al. proposed an intra R-λ, model in [7], and the gradient was used to characterize the picture complexity.
In FIG. 15, the basic process of RC in HEVC is illustrated. Coding tree unit (CTU) level RC is also playing an important role in regulating the bit rate and improving the coding performance. Due to the importance of CTU-level RC, which can greatly influence the Rate-Distortion (R-D) performances, various RC algorithms at CTU level have been proposed for HEVC. An optimized CTU level RC strategy was proposed by Li et al. [8]. Wang et al. [9] proposed a RC scheme based on Lagrange multiplier, which greatly improved the coding efficiency. In [10], Zhou et al. proposed a novel CTU level RC method based on content complexity correlation for HEVC. However, these methods are optimized based on mean square error (MSE), which may not be optimal in terms of perceptual quality. As the ultimate receiver of the video streams is the human visual system [11], it is desirable to have a RC scheme where the perceptual characteristics are fully considered in CTU level bit allocation.